1. Field
This disclosure relates to a calibration light source, in particular using an LED (light emitting diode).
2. Description of the Related Art
The prior art is briefly summarized below.
U.S. 2004/0120156 A1 describes a high-powered spotlight for theatrical lighting or similar in which a thermally conductive board having high-powered LEDs thereon is mechanically coupled to the housing via a thermally isolating support element. The heat generated by the high-powered LEDs passes into the board and is conveyed away into the housing interior by Peltier elements provided in openings in the support element. The board is thermally insulated to prevent thermal back-flow from the inside of the housing to the LEDs and to decouple the LEDs from the housing heat.
U.S. 2008/0285271 describes an illumination source in which dissipation of the heat generated by the LEDs is facilitated by the fact that the inlets into the housing interior are sufficiently large. Thanks to the chimney effect, air convection flow rates through these inlets are high enough to ensure heat exchange into the housing interior.
Calibration light sources are used inter alia for absolute calibration of light or radiation measurement devices such as spectrometers, photometers and radiometers. It is known heretofore that current- and temperature-stabilised reference LEDs may be used as a calibration light source. LEDs are able to emit radiation in the visible, infrared or ultra-violet wavelength range. A semiconductor chip mounted on a circuit board generates the LED radiation. The required wavelengths can be achieved based on the different physical properties of the material used to produce the semiconductor.
One of the key requirements that a calibration radiation source must fulfill is a high degree of stability in spectral radiation distribution and intensity. Hitherto, standard LEDs with a typical operating current of approximately 20 mA have exclusively been used. Constant electrical power UF*IF (UF=forward voltage and IF=forward current) provides the uniform luminous power required in order for the LED to be used as a calibration radiation source. As a general rule, a stable current source is used to provide the current IF. The forward voltage UF which drops across the semiconductor chip is measured at the LED's connectors. Since at constant current IF the forward voltage UF of the LED only varies with temperature, by regulating the temperature of the component it is possible to stabilize the forward voltage UF and hence the luminous output. As a general rule, the operating point at which the LED is operated is approximately 40° C., significantly higher than the ambient temperature. In the case of standard LEDs used hitherto, to maintain a chip temperature of around 40° C. the component has to be heated. Heating elements in the form of heating resistors, heating diodes etc. are usually used to achieve this stabilization. The component, and hence the LED chip, are kept at a constant temperature by measuring the forward voltage UF and modifying the heating power accordingly. A disadvantage of the aforementioned calibration radiation sources lies in the low luminous power that can be achieved with the standard LEDs used.
To ensure a Lambertian radiation pattern, a diffuser is positioned in front of the LED as a cap. The diffuser is inserted into an exterior housing, which protects the calibration radiation source against ambient effects.
However, the ambient temperature also influences the luminous output and the radiation spectrum (LED color) radiated by the calibration radiation source. Hence in prior-art calibration radiation sources, rapid fluctuations in the ambient temperature cannot be compensated sufficiently quickly even with active temperature control of the LED temperature, which results in undesirable temporal variations in the calibration radiation source's luminous intensity.
The object of the present invention is to provide a temperature-stable calibration light source.
This object is achieved via the subject matter set forth in Claim one. Preferred embodiments are described in the independent claims.